In the art, already a number of arrangements and applications for so-called switching sensors are known, generally using magnetic field sensors having an associated signal processing circuit in order to for example perform a rotational speed, position or speed determination of a providing object for example also with a simultaneous detection of the movement direction or rotational direction, respectively, of the providing object or for example also a simple distance recognition of the providing object with respect to the sensor arrangement.
One possibility for the determination of the movement direction or movement speed, respectively, known in the prior art is to use two magnetic field sensor elements which are spatially separated from each other and arranged spaced apart from the providing object to be examined. The sensor element signals of the magnetic field sensor elements are here evaluated separate from each other, wherein from the temporal sequence of the signals of the magnetic field sensor elements, for example using a digital signal processing means DSP, the movement direction or the distance of the providing object may be determined. In such an arrangement now, typically in relation to the providing object, e.g. a gearwheel, a so called backbias magnet is used in order to generate a suitable magnetic field influenced by the different teeth (cams) and depressions of the gearwheel, so that the two spaced apart magnetic field sensor elements may provide different sensor signals depending on the position of the individual teeth and the depressions of the gear wheel.
In FIG. 4 now as an example a schematical illustration of such a Hall sensor arrangement 100 having a switching sensor building block 102 with two Hall elements 104, 106 and an evaluation IC 108 (IC=integrated circuit) is shown as an example. The Hall sensor arrangement 100 further includes a backbias magnet 110 and a gear wheel 112 having teeth 112a (cams) and depressions 112b. The distance L (e.g. 2.5 mm) illustrated in FIG. 4 indicates the distance between the switching sensor building block 102 and the gear wheel 112, the distance a indicates the center distance of the Hall elements 104 and 106 and the distance b indicates the distance of the Hall elements 104, 106 from the exterior housing side of the switching sensor building block 102.
According to the Hall sensor arrangement 100 of FIG. 4 the switching sensor building block 102 detects the movement or the position of a ferromagnetic structure in the form of the teeth 112a and the depressions 112b of the gear wheel 112 by detecting and temporally evaluating the respective magnetic flow density of a magnetic field penetrating the Hall elements 104, 106, which changes according to the respective orientation or position, respectively, of the gear wheel 112. For generating the magnetic field, at the backside of the switching sensor member 102 the so-called backbias magnet 110 including South Pole and North Pole (as indicated) is arranged.
In FIG. 5 now, as an example resulting (idealized) sensor signal courses, i.e. differential signal courses are illustrated, obtained as a differential signal of the sensor output signals of the two Hall elements 104, 106. Here, the signal course 114 indicated as a continuous line is to indicate the differential signal for a large airgap between the switching sensor building block 102 and the gear wheel 112, i.e. for a great distance L, and the signal course 116 of the differential signal indicated as a dashed line the one for a small airgap between the switching sensor building block 102 and the gear wheel 112. It becomes clear that the differential signal courses result depending on the respective difference of the magnetic flow density through the two Hall elements 104, 106. Further, in FIG. 5 an output signal VOUT of the switching sensor building block 102 is illustrated as course 118, wherein it may be seen from FIG. 5, that the output signal VOUT comprises a first high logic signal level (“1”), when the differential signal 114, 116 exceeds a mean value 120 of the differential signal, and comprises a second low logic level (“0”) when the differential signal course 114, 116 falls below the mean value 120 of the differential signal.
In practice, now for example when using a comparator circuit, problems result in so far that in the switching sensor building block the analog input voltage (i.e. the differential signal or simply the sensor signal) is provided with interference signals and noise components. Such interference signals and noise components are not contained in the idealized illustration of FIG. 5. It may further be seen that due to distance changes between the switching sensor building block 102 and the gear wheel 112 (providing object) for example due to vibrations, distance-depending signal deformations result in the differential signal course 114, 116, i.e. the sensor signal. These undesired interference signal components or signal deformations, respectively, in the sensor signal generally cause an undesired switching of the digital output signal 118 (VOUT), which consequently then does not correspond to the actual analog sensor signal anymore. In particular high-frequency interference signals in the analog sensor signal cause a frequent toggling in the comparator circuit and corrupt the digital output signal VOUT. A further problem is the thermal drift in particular of the comparator circuit used for a signal comparison, whereby the accuracy of the mapping of the analog input voltage to the digital output signal VOUT suffers.
FIGS. 6a–6d now show different real or normalized sensor signal courses 114, 116, respectively, determined in practical applications at the camshafts with differently sized airgaps between the switching sensor building block 102 and the camshaft (gear wheel) 102.
FIG. 6a now shows for example the differential signal courses 114, 116 of a camshaft with differently sized airgaps, wherein FIG. 6a shows the situation in which after several teeth a large (long) depression follows. The signal amplitudes are scaled to 100%.
In FIG. 6a clearly different signal courses 114, 116 for a large airgap (signal course 114) and for a small airgap (signal course 116) may be seen, wherein it is in particular obvious in the signal courses illustrated in FIG. 6a that signal course deviations (see arrows in FIG. 6a) and a changed zero passage with regard to the mean value of the signal courses result. It may thus be seen that in the Hall sensor arrangement 100 different output signals result simply based on a changing airgap between the switching sensor building block 102 and the providing object 112, whereby obviously a correct evaluation of the differential signal courses 114, 116 and thus a correct output signal 118 due to signal shape changes is impaired.
FIG. 6b now shows further differential signal courses 114, 116 of a camshaft sensor with different airgaps, wherein the differential signal courses are normalized to 100%. The differential signal course 114 is an example for a small airgap, wherein the differential signal course 116 is indicated as an example for a large airgap.
Also here the already above indicated signal shape changes of the differential signal courses 114, 116 with different airgaps between the switching sensor building block 102 and the providing object 112 (giver object) are obvious, from which again the already above-mentioned difficulties in the evaluation of the differential signal courses 114, 116 or the output signal 118, respectively, result.
FIG. 6c shows as an absolute signal different signal courses of a so called monocell camshaft sensor operating with only one sensor cell with different airgaps, wherein the x axis indicates the angular position, the y axis indicates the absolute signal amplitude, and as parameters the distance between the switching sensor building block 102 and a providing object (camshaft) are indicated. Also here, the signal course 114 indicates a small airgap, wherein the signal course 116 for example indicates a large airgap. Further, intermediate stages of the signal courses 114, 116 are shown.
FIG. 6d shows a relative signal of a camshaft sensor with different airgaps, wherein the signal courses are normalized to 100%, wherein the x axis indicates the angular position, the y axis indicates the relative signal amplitude, and as a parameter the distance between the switching sensor building block and the providing object is indicated. Also here, the signal course 114 indicates a small airgap, wherein the signal course 116 for example indicates a large airgap.
In order to prevent the above-indicated problems with regard to signal shape changes of the signal course or the differential signal course and with regard to interference signals and noise components in the signal courses, it is known to provide a comparator circuit with a hysteresis, i.e. with a top and a bottom switching point. Such comparator circuits are also referred to as so-called Schmitt trigger circuits. In these comparator circuits having two threshold values, two comparators are used whose digital output signals are used for setting and resetting a flip flop. By this, the threshold values or switching levels, respectively, of the comparator circuit may be set particularly accurately. Disadvantageous for comparator circuits having a hysteresis, however, is the technically conditioned falling apart of switch-on and switch-off points.
Switch-on and switch-off point here is the switching of the comparator circuit when exceeding a first top threshold value (first hysteresis threshold) in a first change direction of the analog input signal or the falling below the second bottom threshold value (second hysteresis threshold) in a second change direction of the analog input signal, respectively. In order to now be able to filter out interference signals in the analog input signal, the switch-on and switch-off points of the hysteresis should lie as far apart that interference signals cannot cause a switching of the comparator circuit. In other words, the magnitude of the hysteresis determines the measure of the interference suppression, thereby, however, also the deviation from the desired threshold value.
With switching sensor building blocks using a comparator circuit including hysteresis, e.g. Schmitt trigger circuits, like e.g. gear wheel sensors by means of Hall effect elements or xMR effect elements, an undesired phase error with regard to the analog input signal occurs, i.e. the differential signal course, because the analog input signal is subject to or liable to strong amplitude changes and simultaneously also strong signal shape changes, respectively.
According to the prior art it may be concluded, that elements sensitive with regard to magnetic fields are used whose output signals are evaluated, wherein in particular a signal or a differential signal of the output signal courses is examined by one or several magnetic field detection means according to preset switching thresholds. For evaluating the differential signals, comparator circuits with a hysteresis are used, wherein it is problematic here, however, according to the prior art, that the analog input signal (differential signal course) for example based on positioning changes between the detection means (switching sensor building block 102) and the moving structure (gear wheel 112) is subjected to strong amplitude changes due to distance changes and simultaneously also strong signal shape changes.
Arrangements for considering such amplitude or signal shape changes, respectively, were hitherto realized in the prior art for example by means of circuits for adjusting the switching circuits (threshold adjusting circuit, U.S. Pat. No. 6,064,199), in which the signal amplitude peaks of the differential signal course switch the output of the switching sensor building block. Further, according to the prior art, also switching sensor building blocks were realized in which the switching values, i.e. the hysteresis of the comparator circuits, represent a fixed percentage of the peak to peak voltage value of the analog input signal (fixed percentage of peak to peak voltage, U.S. Pat. No. 5,650,719 and U.S. Pat. No. 6,297,627). Further, for example the U.S. patent application U.S. Pat. No. 5,694,039 describes a proceeding in which the switching signal is applied across an amplifier with a programmable gain or amplification (PGA; PGA=programmable gain amplifier) in the main signal path. Further, according to the prior art, also analog sample and hold circuits for a minimum-maximum amplitude localization are for example proposed in the U.S. patent application U.S. Pat. No. 6,100,680.
In the Hall sensor arrangement known in the prior art using comparator circuits having a hysteresis, it is now disadvantageous that the switching sensor building block switches its output signal based on the determined signal peaks (min or max values, respectively) of the signal course of the sensor signal, wherein for this purpose no sharp signal passages of the analog input signal (the differential signal course) are available. By this, in those known Hall sensor arrangements problems with regard to jitter effects and phase errors result. It should further be noted that a switching sensor building block in which the switching thresholds of the hysteresis represent a fixed percentage of the peak to peak input voltage value and the switching points are adjusted accordingly, undesired phase errors may not be prevented by strong amplitude-dependent signal shape changes.
From the signal courses of FIGS. 6a and 6b normalized to a 100% signal amplitude it may be seen that when using an amplitude-proportional hysteresis (e.g. in the shape of lines A, B), the differential signal courses in a proportionally fixed switching value lead to temporal deviations in switching and thus to phase errors. The deviations of the differential signal course illustrated in FIGS. 6a and 6b by arrows in the y-direction, with proportionally fixed switching points which would be represented as horizontal lines A, B in the diagram, lead to different crossings of the differential signal with those proportionally fixed switching points in the x-direction, as it is represented by lines A, B and the associated arrows in FIG. 6b. 
Basically the same holds true for the signal courses of a monocell sensor illustrated in FIG. 6d. Also here, however, a proportional switching point may be found, by a crossing of the amplitude-dependent signal courses, which are related to 100%, however. If, however, this point is superimposed by a proportionally fixed hysteresis, then it comes to temporally different passage points in the x-direction and thus to phase errors due to the differential signal course in the y-direction. The superimposed proportionally fixed hysteresis could also be represented as horizontal lines A, B here, deviating from this crossing point in the y-direction and causing a switching of the sensor output signal when the signal passes through these horizontal lines A, B, as it is illustrated by the additionally indicated arrows and lines A, B in FIG. 6d. 
Thus, it may be seen from the above-illustrated Hall sensor arrangements according to the prior art that the evaluation of the analog input signal, i.e. the differential signal courses, based on the signals of the Hall elements for determining the position or movement direction, respectively, of the providing object, may not always be performed sufficiently accurately or that this evaluation is very expensive with regard to circuit technology, respectively.